Video: Extraordinary security measures

U.S. officials are taking extraordinary steps to try to thwart a possible attack by al-Qaida terrorists. They even forced French officials to cancel several flights between Paris and Los Angeles.

A senior U.S. official tells NBC News the flights were canceled because of specific intelligence that several flights out of Paris Wednesday could be targeted by terrorists, and that al-Qaida operatives were scheduled to board some flights.

The flights canceled so far are four Air France flights Wednesday and Thursday from Paris to Los Angeles, and two from Los Angeles to Paris.

U.S. officials say the French agreed to cancel the flights after heavy U.S. pressure.

A senior U.S. official says there is also “plausible” intelligence about attacks targeting other flights from other countries in the coming days lasting into January. The United States is already talking to Aero México about canceling flights.

According to terrorism expert Roger Cressey, “We should operate from a presumption that there are additional plots and that the government is going to take action there too.”

Counter-terror sources say there is concern some flight crews may have been infiltrated and arrests are likely. But there is even greater concern about a conventional hijacking.

Additional al-Qaida pilots?NBC News has also learned that a CIA analysis soon after 9/11 estimated that well over a dozen al-Qaida operatives had pilot training. “The threat picture is very disturbing, not just here in the United States but overseas. Multiple locations. Multiple potential axes of targets of attacks,” Cressey adds.

At the Los Angeles Airport, security was raised to its highest level in two years on the basis of specific intelligence about an attack involving explosives.

In Virginia, security was also beefed up around the naval base in Norfolk and the North Anna nuclear power plant — considered the most likely targets near Tappahannock, Va., which was recently mentioned by al-Qaida in intercepted conversations.

“I can’t imagine some guys sitting around a campfire in Afghanistan, saying what harm we can do to the town of Tappahannock, Virginia,” said Tappahannock Police Chief James Barrett.

A French official tells NBC some passengers from the first Paris flight Wednesday have been taken in for questioning. A senior U.S. official says: “There’s no way yet to determine when the worst of this will pass."